首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2984篇
  免费   19篇
  国内免费   1篇
电工技术   5篇
综合类   1篇
化学工业   136篇
金属工艺   3篇
机械仪表   20篇
建筑科学   37篇
矿业工程   1篇
能源动力   16篇
轻工业   153篇
水利工程   13篇
石油天然气   2篇
无线电   78篇
一般工业技术   136篇
冶金工业   2310篇
原子能技术   15篇
自动化技术   78篇
  2021年   9篇
  2020年   11篇
  2019年   14篇
  2018年   16篇
  2017年   16篇
  2016年   7篇
  2015年   11篇
  2014年   17篇
  2013年   31篇
  2012年   42篇
  2011年   36篇
  2010年   28篇
  2009年   30篇
  2008年   41篇
  2007年   43篇
  2006年   27篇
  2005年   32篇
  2004年   22篇
  2003年   20篇
  2002年   20篇
  2001年   21篇
  2000年   23篇
  1999年   72篇
  1998年   722篇
  1997年   380篇
  1996年   240篇
  1995年   164篇
  1994年   156篇
  1993年   152篇
  1992年   32篇
  1991年   41篇
  1990年   27篇
  1989年   32篇
  1988年   32篇
  1987年   33篇
  1986年   33篇
  1985年   39篇
  1984年   10篇
  1983年   13篇
  1982年   16篇
  1981年   11篇
  1980年   34篇
  1979年   9篇
  1978年   12篇
  1977年   51篇
  1976年   121篇
  1975年   11篇
  1974年   7篇
  1973年   5篇
  1970年   5篇
排序方式: 共有3004条查询结果,搜索用时 31 毫秒
61.
Docosahexaenoic acid (DHA), a long-chain omega-3 fatty acid, is important for eye and brain development and ongoing visual, cognitive, and cardiovascular health. Unlike fish-sourced oils, the bioavailability of DHA from vegetarian-sourced (algal) oils has not been formally assessed. We assessed bioequivalence of DHA oils in capsules from two different algal strains versus bioavailability from an algal-DHA-fortified food. Our 28-day randomized, placebo-controlled, parallel group study compared bioavailability of (a) two different algal DHA oils in capsules ("DHASCO-T" and "DHASCO-S") at doses of 200, 600, and 1,000 mg DHA per day (n = 12 per group) and of (b) an algal-DHA-fortified food (n = 12). Bioequivalence was based on changes in plasma phospholipid and erythrocyte DHA levels. Effects on arachidonic acid (ARA), docosapentaenoic acid-n-6 (DPAn-6), and eicosapentaenoic acid (EPA) were also determined. Both DHASCO-T and DHASCO-S capsules produced equivalent DHA levels in plasma phospholipids and erythrocytes. DHA response was dose-dependent and linear over the dose range, plasma phospholipid DHA increased by 1.17, 2.28 and 3.03 g per 100 g fatty acid at 200, 600, and 1,000 mg dose, respectively. Snack bars fortified with DHASCO-S oil also delivered equivalent amounts of DHA on a DHA dose basis. Adverse event monitoring revealed an excellent safety and tolerability profile. Two different algal oil capsule supplements and an algal oil-fortified food represent bioequivalent and safe sources of DHA.  相似文献   
62.
Mixing in co-rotating sinusoidal cavity flows is studied with period, T, and phase shift, a, as parameters. Notice that the main obstacle to uniform mixing is the existence of stable :ower periodic orbits, mixing windows, where uniform mixing takes place, are found in the T— a space u:iing bifurcation analysis. The main advantage of this method is a great saving of the computation time.  相似文献   
63.
L. Zonder  A. Ophir  S. McCarthy 《Polymer》2011,52(22):5085-5091
Different melt mixing sequences were applied to incorporate multiwalled carbon nanotubes (CNTs) into blends prepared from high density polyethylene (PE) and polyamide 12 (PA). Electron microscopy, rheology and electrical resistivity were used to characterize the morphology and microstructure. At a composition of 75PA/25PE, presence of CNT at the interface promoted by premixing the CNTs in the PE phase, resulted in finer phase morphology and a decrease in the resistivity of up to five decades relative to other mixing procedures used. At a composition of 25PA/75PE, premixing the CNT in the PA phase resulted in their segregation inside and around the PA domains and a four decade lower resistivity. Interestingly, compounds that yielded the lowest resistivity were also characterized by increased low frequency melt storage modulus (G′) which indicates the existence of a correlation between the two properties.  相似文献   
64.
Bacteriophages as accessory genetic elements play a crucial role in the dissemination of genes and the promotion of genetic diversity within bacterial populations. Such horizontal transfer of DNA is critical in the emergence of new pathogenic organisms, through the dissemination of genes encoding virulence factors such as toxins, adhesins and agressins. Phages can transfer genes that are not necessary for bacteriophage persistence and are generally recognised by their ability to convert their host bacteria to new phenotypes. This phenomenon is known as phage conversion. If such converting genes encode for virulence factors, the consequences of phage infection may include increased virulence of the host bacteria, and the conversion of a non‐pathogenic strain to a potentially dangerous pathogen. A number of virulence factors in bacteria causing diseases in plants, animals and humans are encoded by converting phages, the vast majority of which are temperate as opposed to lytic in nature. © 2001 Society of Chemical Industry  相似文献   
65.
Conventional test procedures, such as the S4 test to analyze the resistance against rapid crack propagation (RCP) of plastic pipe materials are characterized by usage of a lot of material, are far from saving of time and they are‐in need of special experimental set‐ups. Therefore, in the last decade, small‐scale accelerated reliable tests (SMART) are developed ‐ worldwide to overcome the disadvantage of such conventional tests. In this article, fracture mechanics based analysis of instrumented Charpy impact test data for a set of bimodal high‐density polyethylene pipe grades are compared with data of the conventional Charpy impact test. From this comparison the Charpy impact strength at ?30°C comes forth as a robust reproducible measure of the resistance to RCP and it is therefore proposed as a SMART method to rank materials with respect to RCP resistance. POLYM. ENG. SCI., 57:13–21, 2017. © 2016 Society of Plastics Engineers  相似文献   
66.
The construction of antifouling membranes has been a desirable approach for addressing membrane-fouling issues in the ultrafiltration (UF) process. Antifouling means antiadhesive and antimicrobial; however, few researchers have achieved both properties in a facile and effective manner. In this article, we report a direct tannic acid (TA) coating method combined with the in situ deposition of silver nanoparticles (Ag NPs); this was used to improve the antifouling properties of a positively charged polymeric UF membrane. The results show that the TA–Ag NP modified membranes showed improved protein resistance (flux recovery rate = 71.2% after modification vs 17.8% before modification) and less attachment of bacteria (Escherichia coli K1) on the membrane surface and reduced cell viability in the resulting bacterial suspension (reduced by ≥90%) because of the combined antimicrobial properties of both the TA and Ag NPs. This indicated that our modification method was promising for UF membrane antifouling applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47314.  相似文献   
67.
Almonds are blanched by exposure to hot water or steam-injected water to remove the pellicle (skin) from the kernel. This study evaluated the survival of Salmonella Enteritidis PT 30, Salmonella Senftenberg 775W and Enterococcus faecalis on whole raw almond kernels exposed to hot water. Whole, inoculated (7 to 9 log CFU/g) Nonpareil almonds (40 g) were submerged in 25 L of water maintained at 60, 70, 80 and 88 °C. Almonds were heated for up to 12 min, drained for 2 s, and transferred to 80 mL of cold (4 °C) tryptic soy broth. Almonds in broth were stomached at high speed for 2 min, serially diluted, plated onto tryptic soy and bismuth sulfite agars (Salmonella) or bile esculin agar (Enterococcus) and incubated at 37 °C for 24 and 48 h, respectively. D values of 2.6, 1.2, 0.75 and 0.39 min were calculated for exposure of S. Enteritidis PT 30 to water at 60, 70, 80 and 88 °C, respectively; the calculated z value was 35 C°. D values determined for Salmonella Senftenberg 775W and E. faecalis at 88 °C were 0.37 and 0.36 min, respectively. Neither Salmonella serovar could be recovered by enrichment of 1-g samples after almonds inoculated at 5 log CFU/g were heated at 88 °C for 2 min. These data will be useful to validate almond industry blanching processes.  相似文献   
68.

Objective

To examine the tactics the tobacco industry in Germany used to avoid regulation of secondhand smoke exposure and to maintain the acceptance of public smoking.

Methods

Systematic search of tobacco industry documents available on the internet between June 2003 and August 2004.

Results

In West Germany, policymakers were, as early as the mid 1970s, well aware of the fact that secondhand smoke endangers non‐smokers. One might have assumed that Germany, an international leader in environmental protection, would have led in protecting her citizens against secondhand smoke pollution. The tobacco manufacturers in Germany, however, represented by the national manufacturing organisation “Verband” (Verband der Cigarettenindustrie), contained and neutralised the early debate about the danger of secondhand smoke. This success was achieved by carefully planned collaboration with selected scientists, health professionals and policymakers, along with a sophisticated public relations programme.

Conclusions

The strategies of the tobacco industry have been largely successful in inhibiting the regulation of secondhand smoke in Germany. Policymakers, health professionals, the media and the general public should be aware of this industry involvement and should take appropriate steps to close the gap between what is known and what is done about the health effects of secondhand smoke.  相似文献   
69.
Reliable methods are required for the detection and enumeration of potentially injured E. coli O157 in food in the presence of outnumbering competing bacteria. Selective agents can prevent or inhibit the recovery and subsequent multiplication of injured cells and direct inoculation, either into selective enrichment broths or onto selective agar plates is still used in many methods for E. coli O157 detection and enumeration. When compared with tryptone soya agar (TSA), sorbitol MacConkey agar (SMAC) was shown to underestimate the concentration of viable E. coli O157:H7 subjected to low pH and high NaCl concentration. Using a resuscitation stage on TSA followed by membrane transfer to SMAC improved recovery to levels obtained on TSA. The membrane method was used to monitor the numbers of artificially contaminated E. coli O157:H7 during the fermentation of a meat product and demonstrated better survival when compared to counts on SMAC. Six rapid methods for the detection of E. coli O157 in food (BAX E. coli O157, Reveal 8 E. coli O157-H7 screening test, VIP EHEC, VIDAS E. coli O157 (ECO), EHEC-Tek and Tecra E. coli O157 visual immunoassay), were evaluated using beetburgers, parsley and fermented meat artificially contaminated with injured cells. Methods using direct selective enrichment, with or without an elevated incubation temperature gave false-negative results. The incorporation of a non-selective pre-enrichment medium improved the detection rates of these assays by up to ten fold.  相似文献   
70.
In a field study P and K uptake by two corn (Zea mays L.) genotypes which differed in root growth was investigated. The effect of differences in root growth on P and K uptake was assessed using a mechanistic-mathematical model which describes nutrient uptake by growing plant roots in soil. Nitrogen was applied at 0 and 227 kg ha–1 to Pioneer 3732 and B73xMo17 corn grown on Raub silt loam (fine-silty, mixed, mesic Aquic Argiudoll) and at 227 kg N ha–1 to these two genotypes on Chalmers silt loam (finesilty, mixed, mesic, Typic Haplaquoll). Root growth and P and K uptake by the two corn genotypes was measured 31, 47, 75 and 91 d after planting on the Raub and 31, 47, 61 and 75 d after planting on the Chalmers soil.Root growth and P and K uptake by B73xMo17 was greater than that of Pioneer 3732 on N-fertilized Raub soil. On Chalmers soil the difference in root growth between the two genotypes resulted in an increase in K but not P uptake. The higher soil P level of the Chalmers appears to have offset possible differences in P uptake due to root size. There were no differences between the two genotypes in either the percentage of roots with root hairs, or the density or length of root hairs. Phosphorus and K uptake calculated with the simulation models for both corn genotypes on both soils over each of three growth periods agreed with observed P (Y = 0.68X + 1.71; r = 0.944**) and K (Y = 0.88X + 15.52; r = 0.928**) uptake. Differences in P and K uptake between B73xMo17 and Pioneer 3732 resulted primarily from the difference in root growth in the topsoil. A high correlation was found between root surface area and P (r = 0.893**) and K (r = 0.928**) uptake by both corn genotypes on both the soils.Journal paper No. 10,316 Purdue Univ. Agric. Exp. Stn., W. Lafayette, In 47906. Contribution from the Dep. of Agron. This paper was supported in part by a grant from the Tennessee Valley Authority.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号